In recent years, radio communication systems of OFDM (Orthogonal Frequency Division Multiplexing) system and OFDMA (Orthogonal Frequency Division Multiple Access) system in which data are allocated to a plurality of subcarriers have gained attention. Among them, the OFDMA system has been used, for example, for WiMAX (Worldwide Interoperability for Microwave Access).
In the OFDM system and the OFDMA system, the transmission side transforms a signal containing data to be transmitted as a frequency domain signal into a time domain signal using IFFT (Inverse Fast Fourier Transform) and transmits the transformed signal. The reception side, in contrast, transforms a time domain signal received from the transmission side into a frequency domain signal using FFT (Fast Fourier Transform) and extracts the original data from the frequency domain signal.
Generally, in an OFDM symbol, a cyclic prefix (CP) that is a copy of a trailing portion of an effective symbol containing data to be transmitted is added at the beginning of the effective symbol to prevent the OFDM symbol from being adversely influenced by multi-path interference.
However, the optimum value of the CP length depends on the propagation environment.
For example, in a propagation environment where there is a large delay dispersion, it is necessary to increase the CP length. However, if the CP length is increased, since the amount of data that can be transmitted in a unit time period becomes small, it is improper to increase the CP length in a transmission environment in which a large amount of data is transmitted at high speed.
For example, patent documents 1 and 2 (JP 2005-303826 A and JP 110014 A, Respectively) disclose techniques that can vary the CP lengths of OFDM symbols.
However, in a radio communication system where data are transmitted and received as frames one by one and the frame length is constant, if the CP length is variable and a frame is composed of OFDM symbols, a time period that is shorter than the CP length and the effective symbol length may remain. In this situation, however, the remaining time period becomes an idle time period in which an OFDM symbol cannot be placed and thereby data cannot be transmitted.
Moreover, in a radio communication system where a frame is divided into a plurality of subframes and the subframe length is constant, an idle time period may take place in each subframe. Furthermore, in a radio communication system where the subframe length is variable, an idle time period may take place in a subframe. Thus, to transmit as much data as possible, it is necessary to shorten the idle time period in a frame/subframe.
Thus, in the OFDM system and the OFDMA system where the frame length is constant, even if the CP length is varied, shortening the idle time in a frame/subframe has become an important issue.    [Patent Document 1] JP 2005-303826 A    [Patent Document 2] JP 2005-110014 A